When entering a voucher or an invoice for a project you receive “System Message 12129 ‘Selection is invalid since Multi-Company is not enabled'” after you tab off the project, but the project has a company ID the same as the company you are entering it for.
Go into Project Maintenance | Tasks tab and enter at least one task.
If the project has no tasks set up yet, you must enter at least one in order to enter project-related transactions against it.